Since the availability of 7 GHz of unlicensed spectrum around 60 GHz for wireless personal area wireless network and directional link applications, 77 GHz for adaptive cruise control radar and 94 GHz for image applications, there is growing interest in using this resource for new consumer applications requiring very high-data-rate wireless transmission. The rapid growth of wireless communication is generating demand for integrated planar components to meet various needs, such as size shrink, performance improvement, and cost reduction. Multi-layer configuration makes microwave and RF circuits more compact. This, in turn, opens more opportunity for their use in portable equipments, as well as offering design flexibility.
There have been strong interests in integrating circuits at millimeter wave frequencies using silicon technologies, i.e., system-on-a-chip. However, at frequencies beyond 10 GHz, the applications of silicon based technologies have been largely hindered by the lossy substrate, which results in large parasitic capacitances and high losses in on-chip passive components. The CMOS technologies developed in the last few years present a cost-effective option to realize highly integrated systems combining analog, microwave design techniques, transmission lines and other passive components such as a coupler.
Branchline couplers are used in microwave integrated circuits (MICs), such as balanced mixers, balanced amplifiers and reflection type phase shifters. Conventional couplers tend to consume expensive chip area, especially at low frequencies, thus many efforts have been made to reduce their size. Lumped-element hybrid couplers are ready examples of the attempt to miniaturize couplers, but lumped inductors and capacitors with the required values and high quality factors are not always available for use in MICs. Slow-wave transmission lines are also promising candidates for size reduction.